Napsin A (NAPSA) is a secreted pepsidase produced in type II pneumocytes and renal tubular epithelium. It is involved in processing of surfactant B in the lung and acts as a useful cytoplasmic marker for primary lung adenocarcinomas. When paired with transcription factor TTF-1 and squamous markers TP63 and KRT5 in a marker panel, Napsin A is useful for distinguishing primary lung adenocarcinomas from other metastases or squamous carcinomas. In immunohistochemistry of normal tissue, staining for NAPSA is expected to be cytoplasmic in the lung and kidney. References: Brasch, 2003; Ueno, 2004; Siddiqui, 2012; Ye, 2011; Turner, 2012